Acoustic wave devices using piezoelectric thin film resonators have been used as filters and multiplexers of wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film as disclosed in Japanese Patent Application Publication No. 2005-347898 (Patent Document 1). The region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region. An air gap is located under the resonance region so that the oscillation is not restrained. The air gap is located so as to include the resonance region in plan view.
Since the resonance region is included in the air gap, the strength of the multilayered film including the lower electrode, the piezoelectric film, and the upper electrode in the resonance region is low. Thus, when a stress is applied to the multilayered film, especially the lower electrode of the multilayered film is mechanically broken—for example, a crack is formed.